A transmitter used for telecommunication and broadcasting apparatuses such as a mobile telephone system and a wireless LAN apparatus needs to maintain the waveforms of transmitted signals with high accuracy while keeping its power consumption low. In recent years research and development is underway concerning a digital transmitter employing a combination of a ΔΣ modulator and a Class-D amplifier, which is expected to be a high power efficiency transmitter. Modulation methods for modulation by a ΔΣ modulator of a digital transmitter include envelope ΔΣ modulation, lowpass ΔΣ modulation, and bandpass ΔΣ modulation. These are disclosed in NPL 1-3 and other literature. A digital transmitter employing a combination of a ΔΣ modulator and a Class-D amplifier is disclosed, for example, in PLT 1 and other literature.
A block configuration diagram of a wireless communication device disclosed in PTL 1 is illustrated in FIG. 12. A wireless communication device 900 illustrated in FIG. 12 includes a baseband signal generator 910, an up-converter 920, a Class-D amplifier 930 and a bandpass filer 940. The Class-D amplifier 930 includes a bandpass ΔΣ modulator 931, a drive amplifier 932, a switching voltage controller 933, and a Class-D amplification unit 934.
The wireless communication device 900 generates a baseband signal in the baseband signal generator 910, up-converts the generated baseband signal to a carrier frequency fc in the up-converter 920, and ΔΣ modulates the carrier frequency signal in the bandpass ΔΣ modulator 931 of the Class-D amplifier 930. The ΔΣ modified carrier frequency signal is amplified in the drive amplifier 932 and the Class-D amplification unit 934, and then restored in the bandpass filer 940 and transmitted to the outside as a transmission signal.
In the wireless communication device 900 of PTL 1, the switching voltage controller 933 controls the switching voltage threshold value of the Class-D amplification unit 934 at an optimal value, based on the effective power value of the input signal to the Class-D amplification unit 934. Therefore, even when the drive amplifier 932 is disposed anterior to the Class-D amplification unit 934, deterioration of the EVM (Error Vector Magnitude) of the transmitted signal is reduced.